The proposed research will investigate three hypotheses concerning the regulation of transmission efficacy during prolonged activity. First; does the recycling of neurotransmitters through glial elements play a role in maintenance and plasticity of synaptic transmission? Second; do vesicle membrane recycling and endocytosis occur at these synapses? If so, what is their functional role? If so, what is their functional role? Third; what is the relation of intracellular ion concentration, ionic transport mechanisms and transmitter release processes in controlling synaptic efficacy? Since the crustacean neuromuscular junction provides one of the best models for vertebrate CNS synapses this research may well provide insights into long term control of synaptic transmission and action of centrally acting drugs in areas of the brain which are experimentally inaccessible.